The invention relates to a detection arrangement for the remote detection of targets, comprising a radar signal generator for generating coded RF radar signals, a transmitter unit controlled by the coded RF radar signals, a radar receiver unit and a video processor connected to the radar receiver unit.
A detection arrangement of this type implemented as a radar apparatus is known in many versions. The drawback of a radar apparatus is that the emitted RF waves can be easily detected by means of ESM receivers which reveal the location and usually also the identity of the transmitting object. Besides, once detected, the radar apparatus may be subjected to deliberate interference.
The present invention obviates this drawback and is characterized in that the transmitter unit is designed to transmit optical signals modulated by the coded RF radar signals and that an optical detector is positioned in front of the radar receiver unit for converting echo signals from the transmitted optical signals into coded RF radar signals, possibly containing target information.
The optical signal consequently acts as carrier for the RF radar signals and the original functionality of the radar apparatus is to a large extent retained.
An advantageous embodiment of the invention, which obviates the necessity of using coherent optical signals is characterized in that the transmitter unit is designed for the amplitude modulation of the optical signals.
An incidental advantage of this embodiment is that the optical detector can be implemented as a relatively low-cost envelope detector.
A detection arrangement according to the invention is eminently suitable for Doppler processing, in which it is not the Doppler shift of the optical signals that is utilized, but the Doppler shift of the coded RF radar signals. A further fact is that a target""s radar cross-section is mainly determined by the wavelength of the optical signals. This provides an additional degree of freedom; if, for instance, the dispersion of a gas cloud is to be followed, the RF frequency should be selected such that a good Doppler processing is ensured and optical signals that provide a good reflection on the gas cloud.
A further advantageous embodiment of the invention is characterized in that the signal generator is designed to generate pulse bursts. The use of pulse bursts in radar is well-known, just like the fact that this yields an optimal Doppler processing. The disadvantage of radar is that a burst can easily be jammed by a repeater jammer, which does not apply to the detection arrangement according to the invention, because optical repeater jammers are not available.
A further advantageous embodiment of the invention, which makes optimal use of the pulse bursts, is characterized in that the video processor includes a Doppler processor of the moving target indication (MTI) or moving target detection (MTD) type to enable the detection of moving targets.
A further advantageous embodiment of the invention is characterized in that the signal generator is designed to generate FMCW radar signals and that the receiver comprises a mixer stage for combining the echo signals detected by the optical detector with signals generated by the signal generator. The detection arrangement will then incorporate all the advantages normally inherent in an FMCW radar apparatus, but is virtually undetectable. Besides, the problem involving the isolation between the transmitter unit and the receiver unit can be solved more easily, because the actual radar beam is then generated by means of optical components.
A further advantageous embodiment of the invention is characterized in that the optical detector incorporates at least two optical diodes for producing RF monopulse signals and that the radar receiver unit is a monopulse receiver. Thus, the direction of a target can be determined on the basis of the phase or amplitude difference between the RF radar signals. In this embodiment, with two diodes disposed in the horizontal plane, the invention can be used to advantage as a collision avoidance radar for vehicles.
A further advantageous embodiment of the invention is characterized in that the optical detector incorporates four optical diodes for producing an RF sum signal, an RF xcex94-azimuth difference signal and an RF xcex94-elevation difference signal. In this embodiment, the detection arrangement is equivalent to a monopulse fire control radar well-known in the art, on the understanding that the arrangement is now capable of tracking an aircraft without the aircraft""s lock-on warning system being activated.
A further advantageous embodiment of the invention is characterized in that the detection arrangement is designed for underwater applications. This is feasible because a wavelength of the optical signals can be chosen such that a good penetration depth can be attained. This makes it possible to detect the approach of, for instance, torpedoes on the basis of their Doppler velocity. Mounted underneath a helicopter for instance, the detection arrangement would make it possible to locate objects below the water surface and, for instance, to track a submarine on the basis of its Doppler velocity.
As the general radar equation is in fact relevant to the detection arrangement according to the invention, an increase of the effective range can be realized by either increasing the transmitted power or by enhancing the receiver unit sensitivity. An advantageous embodiment of the invention is characterized in that a light amplifier is positioned in front of the optical detector so that the transmitted power can be kept at a relatively low level.